but my problems are little more tough.I should not use myscanf(char *s,char *fmt,...) Also it should support %c,%x and %s for strings too

Could we please see some of your code?
I've seen this on another forum, and I've yet to see any code from you.
People are not just going to do it for you (hopefully).
If you want to cheat, look up some open source code for scanf.